Technical Field
The present disclosure relates to a MEMS (Micro-Electro-Mechanical System) acoustic transducer of a differential type.
Description of the Related Art
As is known, a MEMS acoustic transducer, for example a microphone of a capacitive type, generally comprises a micromechanical detection structure, which is designed to transduce acoustic pressure waves into an electrical quantity (in particular a capacitive variation), and an electronic reading interface, which is designed to carry out appropriate processing operations (amongst which amplification and filtering operations) on the same electrical quantity to provide an electrical output signal (for example, a voltage).
The micromechanical structure in general comprises a mobile electrode, provided as a diaphragm or membrane, arranged facing a fixed electrode, at a small distance of separation (the so-called “air gap”), for providing the plates of a detection capacitor with capacitance that is variable as a function of the acoustic pressure waves to be detected. The mobile electrode is generally anchored, by a perimetral portion thereof, to a fixed structure, whereas a central portion thereof is free to move, or undergo deformation, in response to the pressure exerted by the incident acoustic waves, thus causing a capacitance variation of the detection capacitor.
By way of example, FIG. 1 shows a micromechanical structure 1 of a MEMS acoustic transducer, of a known type, which comprises a structural layer, or substrate, 2 of semiconductor material, for example silicon, in which a cavity 3 is provided, for example via chemical etching from the back. A membrane, or diaphragm, 4 is coupled to the structural layer 2 and closes the cavity 3 at the top; the membrane 4 is flexible and, in use, undergoes deformation as a function of the pressure of incident acoustic waves.
A rigid plate 5 (generally known as “back plate”) is arranged facing the membrane 4, in this case above it, via interposition of spacers 6 (for example, of insulating material, such as silicon oxide). The back plate 5 constitutes the fixed electrode of a variable-capacitance detection capacitor, the mobile electrode of which is constituted by the membrane 4, and has a plurality of holes 7, which are designed to enable free circulation of air towards the same membrane 4 (rendering the back plate 5 in effect “acoustically transparent”).
The micromechanical structure further comprises (in a way not illustrated) membrane and rigid-plate electrical contacts, used for biasing the membrane 4 and the back plate 5 and acquiring a signal representing the capacitive variation that results from deformation of the membrane 4 caused by the incident acoustic pressure waves. In general, these electrical contacts are arranged in a surface portion of the die in which the micromechanical structure is made.
As is known, the sensitivity of the MEMS acoustic transducer depends, amongst other factors, upon the mechanical characteristics of the membrane 4 of the micromechanical structure, in particular upon its dimensions, for example in terms of surface area, and upon its electrical biasing.
Typically, the micromechanical structure of the MEMS acoustic transducer is charge-biased. In particular, a DC biasing voltage is applied, usually from a charge-pump stage (the higher this voltage, the higher the sensitivity of the microphone), and a high-impedance element (with impedance of the order of teraohms, for example between 100 GΩ and 100 TΩ) is inserted between the charge-pump stage and the micromechanical structure.
This high-impedance element is usually provided by a pair of diodes in back-to-back configuration, i.e., connected together in parallel, with the cathode terminal of one of the two diodes connected to the anode terminal of the other, and vice versa, or by a series of pairs of diodes once again in back-to-back configuration. The presence of this high impedance “isolates” the DC charge stored in the micromechanical structure from the charge-pump stage, at frequencies higher than a few hertz.
Since the amount of charge is fixed, an acoustic signal (acoustic pressure) that impinges upon the membrane 4 modulates the gap with respect to the back plate 5, producing a corresponding capacitive variation and a consequent voltage variation.
This voltage is detected by an electronic interface circuit with a high input impedance (in order to prevent the charge stored in the micromechanical structure from being perturbed) and then converted into a low-impedance signal (designed to drive an external load).
FIG. 2 shows a possible embodiment of the electronic interface circuit, designated by 10, in this case with single output, namely, a so-called “single-ended” circuit; the micromechanical structure 1 of the MEMS acoustic transducer is represented schematically as a detection capacitor 12 with capacitance CMIC that varies as a function of the acoustic signal detected.
The letter “m” designates, in FIG. 2 (and in the subsequent figures), the membrane 4 of the micromechanical structure 1. Given that, typically, the membrane 4 has a high parasitic capacitance in regard to the substrate 2 (comparable with the capacitance of the detection capacitor of the micromechanical structure itself), whereas the back plate 5 has a lower parasitic capacitance, the membrane 4 is electrically connected to a first low-impedance node N1, for example to a ground operating voltage of the circuit, in order to prevent any attenuation of the signal, whereas the back plate 5 is electrically connected to a second node N2, on which the detection signal that is indicative of the capacitive variations of the detection capacitor is acquired.
The second node N2 is further electrically connected to a charge-pump stage (not illustrated herein), by interposition of a first isolating element 13, having a high impedance, constituted by a pair of diodes in back-to-back configuration, in order to receive a biasing voltage VCP.
The interface circuit 10 further comprises a decoupling capacitor 14, having capacitance CDEC, and an amplifier 15, in buffer or voltage-follower single-ended configuration (i.e., with the inverting input connected to the single output).
The decoupling capacitor 14 is connected between the second node N2 and the non-inverting input of the amplifier 15, which further receives an operating voltage VCM from an appropriate reference-generator stage (not illustrated herein), via interposition of a second isolating element 16, with high impedance, constituted by a respective pair of diodes in back-to-back configuration.
The operating voltage VCM is a DC biasing voltage, appropriately chosen for setting the operating point of the amplifier 15. This operating voltage VCM is chosen, for example, in an interval comprised between a supply voltage VDD and the ground reference voltage. During operation of the MEMS acoustic transducer, the (AC) detection signal is thus superimposed on the DC operating voltage VCM.
The amplifier 15 provides on the single output an output voltage VOUT, as a function of the signal detected by the micromechanical structure 1 of the MEMS acoustic transducer.
This single-ended circuit configuration has some drawbacks, amongst which poor rejection in regard to any common-mode disturbance component, for example deriving from the supply noise or from crosstalk, due to near devices having time-varying signals.
In order to overcome the above drawbacks, the single-ended solution may be replaced by a differential configuration, which should theoretically afford a higher signal-to-noise ratio (SNR).
As illustrated in FIG. 3, the interface circuit 10 in this case comprises a so-called “dummy” capacitor 22, with capacitance CDUM, having a nominal value equal to the value of capacitance at rest (i.e., in the absence of external stresses) CMIC of the detection capacitor 12 of the micromechanical structure 1.
Furthermore, the interface circuit 10 comprises a differential amplifier 25 with four inputs and two outputs, the so-called “fully balanced differential difference amplifier” (FDDA or FBDDA), having a fully differential architecture and a unity gain.
In particular, the second node N2 of the detection capacitor 12 is in this case connected, via interposition of the decoupling capacitor 14, to a first non-inverting input 25a of the differential amplifier 25, a first inverting input 25b of which is directly connected in feedback mode to a first output terminal Out1.
Likewise, the dummy capacitor 22 has a respective first node, designated by N1′, connected to the ground terminal, and a second node N2′ connected, via interposition of a respective decoupling capacitor 24, to a second inverting input 25c of the differential amplifier 25, a second non-inverting input 25d of which is further directly feedback-connected to a second output terminal Out2 (output voltage Vout is present between the first and second output terminals Out1, Out2).
The respective second node N2′ of the dummy capacitor 22 further receives the biasing voltage VCP through a respective first isolating element 23, which is constituted by a pair of diodes in back-to-back configuration and receives the biasing voltage VCP. Likewise, the second inverting input 25c receives the operating voltage VCM, via a respective second isolating element 26, with high impedance, in the example also being constituted by a pair of diodes in back-to-back configuration (the operating voltage VCM is thus a biasing voltage common for the first non-inverting input 25a and the second inverting input 25c of the differential amplifier 25).
The dummy capacitor 22, in this case, enables creation of a substantially balanced path for the buffer inputs (i.e., the non-inverting input 25a and the inverting input 25c) of the differential amplifier 25, for a better common-mode rejection of the disturbance or noise.
Even though the differential configuration described with reference to FIG. 3 enables improvement of the disturbance rejection capacity, not even this makes it possible to increase the signal-to-noise ratio SNR as desired.
In general, the need is thus felt to provide an electronic interface circuit for a MEMS acoustic transducer enabling the signal-to-noise ratio (SNR) to be increased, without at the at the same time varying the sensitivity of the transducer, defined as the variation of voltage at output from the interface circuit, for an increase of the sound pressure level of 1 pascal (Pa). It should be noted that the latter characteristic implies that the signal generated by the MEMS acoustic transducer remains substantially the same, whereas the intrinsic noise of the same transducer is reduced, this being in general difficult to obtain, since MEMS sensors are generally designed to provide the maximum signal-to-noise ratio (SNR).